Injection molding method for cup-shaped container, mold used therefor, and cup-shaped container manufactured by the method

Abstract

To provide an injection molding method for a cup-shaped container, capable of preventing an occurrence of molding defects such as short shots by making a molten resin reach an end of a cavity space evenly without causing stagnation in a flow of the molten resin in a cavity space for molding a body part, which is a narrow space, and a mold used therefor.SOLUTION: A reservoir part is created near an end of a cavity space 14 for molding a body part, bulging toward a cavity-side mold, and a molten resin that flows through the cavity space 14 and reaches the reservoir part first is temporarily slowed down in the reservoir part to equalize a circumferential flow rate, thereby allowing the molten resin to reach the end of the cavity space 14 evenly.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for injection molding a cup-shaped container, a mold used for that method, and a cup-shaped container manufactured by that method. More specifically, the present invention relates to a method for injection molding a relatively tall cup-shaped container without causing molding defects such as short shots at the ends, a mold used in that method, and a cup-shaped container manufactured by that method. [Background technology]

[0002] Needless to say, injection molding is a molding method in which molten resin is filled into the cavity of a mold, the resulting molded product is cooled and solidified inside the mold, and then the mold is opened and removed. Filling the cavity with molten resin is done in two stages: the injection process and the pressure-holding process. The molten resin enters the mold from the nozzle and is supplied to the gate either directly from the sprue or from the sprue via a runner to fill the cavity.

[0003] Although the molten resin is filled into the cavity as described above, depending on the shape of the cavity, particularly its width, it may not reach the ends evenly, resulting in molding defects known as short shots. Recently, in response to demands for lighter weight and thinner walls, many thin-walled cup-shaped containers have been sold. However, when these types of thin-walled containers are injection molded using a high-speed injection molding machine to increase production efficiency, molding defects such as short shots and flash are likely to occur.

[0004] Specifically, when injection molding a thin-walled cup-shaped container, a gate is typically provided at the center of the container's bottom. The molten resin injected through the gate spreads radially, enters the cavity for molding the body, and flows toward its end. However, in the case of a mold for molding a thin-walled cup-shaped container, the cavity for molding the body itself is narrow, and after a skin layer forms where the molten resin contacts the mold surface, the flow layer narrows further as solidification progresses. As a result, a difference occurs between the filling pressure near the gate and the filling pressure at the end of the cavity for molding the body. This makes it difficult for the molten resin to reach the end of the cavity evenly, making short shots more likely to occur.

[0005] Among the above-mentioned thin-walled cup-shaped containers, in the case of flanged containers in which the body is thin-walled and the flange is thicker than the body to provide strength, there is a flange-molding cavity space beyond the body-molding cavity space, and the molten resin flows from the body-molding cavity space toward the flange-molding cavity space, making it even more difficult for the molten resin to reach the ends of the cavity space evenly, making short shots more likely to occur.

[0006] Furthermore, when molding a tall, cup-shaped container such as a drinking container, the width of the cavity space should be uniform all around, but if the core mold is even slightly misaligned within the cavity mold, the cavity space, in other words, the width of the flow layer between the two molds, will become uneven, and the molten resin injected from the gate will flow unevenly to the wider side, and the molten resin that reaches the end will wrap around to the narrower side on the opposite side and rise, creating a gap (hole) between it and the molten resin that flows down later. This phenomenon can occur whether the container has a flange or not. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 3542123 Summary of the Invention [Problem to be solved by the invention]

[0008] As described above, when high-speed injection molding a cup-shaped container with a thin body, the cavity space for molding the body is narrow, and the fluidized layer becomes even narrower as the skin layer forms and solidifies, which reduces the flow rate of the molten resin. If the width of the fluidized layer becomes uneven, the molten resin will drift and not reach the ends of the cavity evenly, which may result in molding defects such as short shots. Such molding defects are particularly common in flanged containers, which have thin bodies and thicker flanges than the body and are in high demand these days.

[0009] The present invention has been made to solve the problems associated with conventional high-speed injection molding of thin-walled cup-shaped containers, and its objective is to provide a method for injection molding a cup-shaped container, a mold to be used therefor, and a cup-shaped container manufactured by this method, which can ensure that the molten resin reaches the ends of the cavity space evenly without causing any stagnation in the flow of the molten resin in the narrow cavity space used to mold the body portion, thereby preventing molding defects such as short shots. [Means for solving the problem]

[0010] The invention described in claim 1 for solving the above problem is a method for injection molding a cup-shaped container having a body and a bottom, comprising: This is a method for injection molding a cup-shaped container, characterized in that a reservoir section is created near the end of the cavity space for molding the body section, bulging toward the cavity-side mold, and the molten resin flowing within the cavity space and reaching the reservoir section first is temporarily slowed down in the reservoir section, thereby leveling the flow rate of the molten resin at various points in the circumferential direction, thereby allowing the molten resin to reach the end of the cavity space evenly.

[0011] In one embodiment, the reservoir is formed continuously or discontinuously, and is formed within a range of one-third of the total height of the cavity space for molding the body portion from an end of the cavity space for molding the body portion. In another embodiment, the cup-shaped container is a flanged container having a flange that is thicker than the body portion.

[0012] The invention described in claim 5 for solving the above problem is a mold for injection molding a cup-shaped container, comprising: This is a mold for injection molding cup-shaped containers, characterized in that a shallow annular recess is formed near the end of the cavity space forming surface for molding the body portion of the cavity-side mold.

[0013] In one embodiment, the annular recess is formed continuously or discontinuously across the wall. Also, in another embodiment, the annular recess is formed within a range from an end of the body-molding cavity space-forming surface to one-third of the total height of the body-molding cavity space-forming surface. Also, in another embodiment, the cup-shaped container is a flanged container having a flange thicker than the body.

[0014] The invention set forth in claim 9, which is intended to solve the above-mentioned problems, is a cup-shaped container manufactured by the injection molding method for a cup-shaped container set forth in any one of claims 1 to 4.

[0015] The invention set forth in claim 10 for solving the above problem is a cup-shaped container manufactured using the injection molding die for cup-shaped containers set forth in any one of claims 5 to 8. [Effects of the Invention]

[0016] The injection molding method for cup-shaped containers and the mold used therein according to the present invention are as described above. With this method and mold, during high-speed injection molding of cup-shaped containers, particularly cup-shaped containers with thin-walled bodies, a reservoir portion is created near the end of the cavity space used to mold the body, bulging out toward the cavity-side mold. Of the molten resin flowing within the cavity space, the molten resin that reaches this reservoir portion first temporarily slows down in this reservoir portion, thereby leveling out the flow rate at various points in the circumferential direction. This allows the molten resin flowing within the cavity space used to mold the body to reach the end smoothly and evenly, resulting in the prevention of molding defects such as short shots. [Brief explanation of the drawings]

[0017] [Figure 1] 1A to 1C are diagrams showing examples of shapes of flanged containers manufactured by the injection molding method for cup-shaped containers according to the present invention. [Figure 2] 1 is a cross-sectional view of a main part of a flanged container manufactured by the injection molding method for a cup-shaped container according to the present invention. [Figure 3] 1 is a partial cross-sectional view showing an example of the shape of an injection molding die for a cup-shaped container according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention relates to a method for injection molding a cup-shaped container having a body and a bottom, and is characterized in that a reservoir portion is formed near an end of a cavity space for molding the body portion, the reservoir portion bulging toward a cavity-side mold, and the velocity of the molten resin flowing through the cavity space and reaching the reservoir portion first is temporarily reduced in the reservoir portion to equalize the flow velocity in the circumferential direction, thereby allowing the molten resin to reach the end of the cavity space evenly.

[0019] 3, the injection mold for cup-shaped containers according to the present invention comprises a cavity-side mold 11 and a core-side mold 13, and is characterized in that a shallow annular recess 12 is formed near the end of the surface of the cavity-side mold 11 that forms the cavity space for molding the body portion. The annular recess 12 is formed continuously or discontinuously across the lateral side, and is located within one-third of the total height from the end of the surface of the cavity-side mold 11 that forms the cavity space for molding the body portion, for reasons that will be described later.

[0020] In FIG. 3, the gaps between the body molding cavity space 14 and the flange molding cavity space 15 are shown at approximately the same intervals, but this is to clearly show the body molding cavity space 14 and does not correspond to the actual size.

[0021] 1 is a diagram showing an example of the shape of a flanged container 1 injection-molded by the method according to the present invention using a mold according to the present invention. This flanged container 1 comprises a relatively tall, bottomed body 2 and an outward flange 3 connected to the upper end of the body 2, with the flange 3 being molded to be thicker than the body 2. In this case, for example, the thickness of the body 2 is set to a range of 0.1 to 1.0 mm (preferably 0.2 to 0.4 mm), and the flange 3 is about twice as thick. The body 2 is formed in a cylindrical or rectangular tube shape with a slightly upward opening to enable stacking.

[0022] The body portion 2 has a thick-walled portion 4 near the flange 3 on its upper outer periphery (see FIG. 2). The thick-walled portion 4 is formed by an annular recess 12 in accordance with the above-described method of the present invention, and is typically formed as a continuous horizontal section, but may also be formed discontinuously. The vertical cross section of the thick-walled portion 4 may be, for example, a mountain-like shape with a low peak and a long base, as shown in the figure, with a vertical width of 7 to 10 mm and a maximum thickness (the distance from the inner surface of the body portion 2 to the peak) of 1.2 to 10 times the thickness of the body portion 2. The vertical cross-sectional shape of the thick-walled portion 4 is not limited to that shown in the figure, and may be any shape as long as it does not interfere with mold opening.

[0023] As described above, the annular recess 12 is positioned within one-third of the total height from the end of the surface forming the cavity space for molding the body portion, and therefore the thick-walled portion 4 is formed within the upper one-third of the total height of the body portion 2 (see FIGS. 1 and 2). If the annular recess 12 were positioned in the middle of the surface forming the cavity space for molding the body portion or in the upper part close to the gate 6, the fluidity of the molten resin, the flow rate of which has been adjusted over the entire circumference as described below, would be more likely to vary in the part close to the end of the cavity space 14 for molding the body portion (the part close to the cavity space 15 for molding the flange), and the effect intended by the present invention would be diminished.

[0024] An auxiliary annular recess may be formed in the core-side mold 13 at a portion corresponding to the annular recess 12 of the cavity-side mold 11 to create an auxiliary reservoir for receiving the flow end of the molten resin. In this case, a continuous or discontinuous annular protrusion 5 is formed on the inner surface of the body 2 at a portion corresponding to the thick-walled portion 4 (see FIGS. 1 and 2). The annular protrusion 5 is formed in a thin, kamaboko shape with a cross section of, for example, about 1.2 mm wide and 0.2 mm high so as not to interfere with stacking.

[0025] When a flanged container is manufactured using the method and mold of the present invention, the molten resin supplied from the gate 16, which is typically located in the center of the bottom forming section, spreads radially into the body molding cavity space 14 and flows toward its end.

[0026] However, when high-speed injection molding a cup-shaped container with a thin body 2, the body-molding cavity space 14 is narrow, and the fluidized layer narrows further as the skin layer forms and solidifies, causing the flow rate of the molten resin to decrease and the flow rate to vary at various points in the circumferential direction. As a result, the flow ends of the molten resin do not flow in unison, resulting in some areas descending quickly and others descending slowly. As a result, the molten resin does not reach the end of the body-molding cavity space 14 evenly, causing molding defects such as short shots. This risk increases when high-speed injection molding a flanged container 1 in which the flange 3 is thicker than the body 2.

[0027] However, with the method and mold of the present invention, the annular recess 12 formed in the cavity-side mold 11 creates a reservoir in the body-molding cavity space 14, and the flow speed of the molten resin that reaches the reservoir first among the molten resin flowing within the cavity space 14 temporarily decreases there. This equalizes the flow speed of the molten resin at various points in the circumferential direction, and thereafter the molten resin flows almost uniformly in unison, spreading evenly all the way to the end of the body-molding cavity space 14. This eliminates the risk of molding defects such as short shots. [Industrial Applicability]

[0028] The injection molding method for cup-shaped containers and the mold used therein according to the present invention are as described above. With this method and mold, during high-speed injection molding of cup-shaped containers, particularly cup-shaped containers with thin-walled bodies, a reservoir portion is created near the end of the cavity space used to mold the body, bulging out toward the cavity-side mold. Of the molten resin flowing within the cavity space, the molten resin that reaches this reservoir portion first temporarily slows down in this reservoir portion, thereby leveling out the flow rate at various points in the circumferential direction. This allows the molten resin flowing within the cavity space used to mold the body to reach the end smoothly and evenly, thereby preventing molding defects such as short shots. Therefore, the present invention has great industrial applicability. [Explanation of symbols]

[0029] 1. Flanged vessel 2. Torso 3 flange 4 Thick part 5 Auxiliary space 11 Cavity side mold 12 Annular recess 13 Core side mold 14 Body molding cavity space 15 Flange molding cavity space 16 Gates

Claims

1. 1. A method for injection molding a cup-shaped container having a body and a base, comprising: A method for injection molding a cup-shaped container, characterized in that a reservoir section is created near the end of the cavity space for molding the body section, bulging toward the cavity-side mold, and the molten resin flowing within the cavity space and reaching the reservoir section first is temporarily slowed down in the reservoir section, thereby leveling the flow rate of the molten resin at various points in the circumferential direction, thereby allowing the molten resin to reach the end of the cavity space evenly.

2. The method for injection molding a cup-shaped container according to claim 1 , wherein the reservoirs are formed in a horizontally continuous or discontinuous manner.

3. 2. The method for injection molding a cup-shaped container according to claim 1, wherein the reservoir is formed within a range of one-third of the total height of the cavity space for molding the body portion from an end of the cavity space for molding the body portion.

4. 2. The method for injection molding a cup-shaped container according to claim 1, wherein the cup-shaped container is a flanged container having a flange portion that is thicker than a body portion.

5. A mold for injection molding a cup-shaped container, 1. A mold for injection molding a cup-shaped container, characterized in that a shallow annular recess is formed near the end of the cavity space forming surface of the cavity side mold for molding the body portion.

6. 6. The injection mold for forming a cup-shaped container according to claim 5, wherein the annular recess is formed horizontally continuously or discontinuously.

7. 6. The injection molding die for cup-shaped containers according to claim 5, wherein the annular recess is formed within a range of one-third of the total height of the cavity space forming surface for molding the body portion from the end of the cavity space forming surface for molding the body portion.

8. 6. The injection mold for a cup-shaped container according to claim 5, wherein the cup-shaped container is a flanged container having a flange portion that is thicker than a body portion.

9. A cup-shaped container manufactured by the injection molding method for a cup-shaped container according to any one of claims 1 to 4.

10. A cup-shaped container manufactured using the injection molding die for cup-shaped containers according to any one of claims 5 to 8.

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